In the automobile industry, home electronics industry, construction industry, and the like, a press-formed closed-structure part has been manufactured by separately forming a pair of parts each having a substantially angular-U-shaped cross section and flanges at both end surfaces thereof in the cross-sectional direction; and by joining these parts together at the flanges by spot welding or continuous welding such as laser welding, to make the closed-structure part, which is a product.
To manufacture such a press-formed closed-structure part at a low cost while reducing the weight and increasing the impact-absorbing ability and rigidity of the press-formed part, methods of press-forming a single blank into a closed-structure part having a polygonal cross section have been proposed.
Japanese Unexamined Patent Application Publication No. 2010-115674 describes a method that supports the central portion of a material and press-forms a flat plate-shaped material into a shape that has a curvature when an end portion of the material is seen in a plan view and that has flange surfaces below side wall surfaces when the material is seen in a side view. Moreover, JP '674 describes that a difference in the line lengths at the end portions of the material before and after bending is substantially eliminated and an occurrence of wrinkling in the formed portion is suppressed by providing the side wall surfaces of the press-formed body with protruding beads and providing the flange surfaces directly below the side wall surfaces with recessed beads.
Japanese Unexamined Patent Application Publication No. 2008-200688 describes a press-forming apparatus that press-forms a flat plate-shaped material plate into a product that includes a curved portion curved along the longitudinal direction, extension portions extending from the curved portion in the longitudinal directions, and flanges extending sideways from the curved portion and the extension portions. The press-forming apparatus includes pressing portions to press edge portions of the material plate, which form the flanges of the curved portion, and driving means to move extension-portion forming dies in directions that residual stresses generated in the flange surfaces of the curved portion is cancelled out.
When a press-formed part having a curved surface is formed by using the technology described in JP '674, if an expression D1-D2 has a positive value when a line length (D1) of a blank-flange-corresponding portion is compared with a line length (D2) of the corresponding portion after being press-formed, or a line length before the press-forming is greater than a line length after the press-forming, wrinkling and buckling are likely to occur. To eliminate the difference in the line lengths, JP '674 provides the side wall portions with protruding beads and provides the flange surfaces directly below the side wall portions with recessed beads.
However, according to JP '674, the recessed/protruding shapes can be formed only in the side wall portions of the part and on the flange portions that are continuous with the side wall portions due to a limitation on the structure of the die. Therefore, the method of JP '674 can be used only for a part that has a curvature in a plan view (that is, a press-formed part having a linear shape in a side view). The method has another problem in that it cannot be used for a closed-structure part formed from a single blank.
JP '688 discloses a method of reducing the residual stress generated in the flange surfaces and increasing dimensional precision by press-forming a part that has a curvature in a plan view while applying a compressive load in the longitudinal directions. The method of JP '688 also has a problem in that it can be used only for a part that has a curvature in a plan view (that is, a press-formed part having a linear shape in a side view).
As described above, with existing methods, it is not possible to easily press-form a curved closed-structure part that has a curved surface at least in the bottom portion thereof.
It could therefore be helpful to provide a way to manufacture with high dimensional precision a press-formed curved closed-structure part that has a curved surface in the bottom portion thereof while reducing a manufacturing cost by reducing the number of forming steps and the number of dies.